Interactive wireless telecommunication services are developing ever more rapidly. These services relate in particular to telephony, telefax, television, the Internet network and any so-called multimedia domain. The equipment for these general-broadcast services have to be available at reasonable cost. This is true in particular for the user's transmission/reception system which has to communicate with a server, usually by way of a telecommunication satellite. In this case, the communications are performed in the microwave frequency domain, especially in the C, Ku or Ka bands, that is to say at frequencies lying between 4 GHz and 30 GHz.
For the transmission (T)/reception (R) source antennas, use is usually made of waveguide devices generally comprising a wide frequency band corrugated horn so as to cover the two bands, transmission and reception, this horn being associated with a device allowing the separation of the transmission and reception paths and/or the orthogonal polarizations and which consist of an orthomode (or OrthoMode Transducer: OMT) and of waveguide filters on each of the ports.
The implementational technology is unwieldy and expensive. Its weight and bulk are generally incompatible with use by individuals.
Thus, the applicant has already proposed in Patent WO99/35711 in the name of THOMSON Multimedia a transmission/reception source-antenna situated at the focus of a focusing system, such as a spherical lens, a parabolic-reflector antenna or a multireflector antenna, which may be used in home terminals for satellite communication systems. In this case, the source-antenna used for illuminating the lens or the parabolic reflector consists of an array of N radiating elements, i.e. of N patches for one direction of link such as reception and of a longitudinal-radiation antenna such as a helix, a dielectric rod, with axis coinciding with the axis of radiation or any other type of longitudinal-radiation antenna for the other direction of link for example transmission, this antenna being situated at the centre of the array. Thus the phase centres of the longitudinal-radiation antenna and of the array of patches practically coincide and can be placed at the focus of the system of antennas.
In order for this type of mixed source to ensure maximum decoupling between the array of N radiating elements of patch type and the longitudinal-radiation antenna such as a helix, it is preferable for the array of patches to be used for the link effected at low frequency, i.e. in reception, and for the longitudinal-radiation antenna to be used for the link effected at high frequency, i.e. in transmission.
However, the reception frequency band generally being wider than the transmission frequency band and the link budget being more sensitive to losses of the reception source, the choice of an array of patches for the reception source is not optimal from this point of view.
Moreover, with an array of patches, it is often difficult to obtain circular polarization of good quality throughout the reception band. However, most communication systems using low-orbit satellites operate with circular polarizations.
The aim of the present invention is therefore to propose an optimal solution to the problems hereinabove, in the case of satellite communication systems using circular polarizations.